There are many chemical processes where it is necessary to bring into contact a gas and solid particulate matter, or solids, or particles. Frequently, chemical reactions as well as physical phenomena take place during such contact. In most cases, gas and solids must be in contact for a minimum time period and the desired chemical or physical reaction or change will not take place or will be incomplete if the contact is for a shorter period. In some cases there is a maximum contact time period, beyond which less than optimum or undesirable results will be obtained. It is highly desirable to conduct gas/solid contacting processes in a continuous or semi-continuous manner rather than as a batch operation.
A contacting zone is usually maintained at some positive pressure (above atmospheric) of the contacting gas. Particles must be introduced and withdrawn from the pressurized zone without losing contacting gas to the atmosphere. It is often necessary to maintain the internal pressure of the contacting zone at a particular value or within a certain range. Contacting zone pressure may be higher than that of the zone from which solids are provided to the contacting zone. Feeding solids into a zone against a high pressure poses numerous problems. When equipment, such as screw conveyors or star valves is used, contact between equipment and solids degrades the solids particles by breaking them into smaller particles and causes equipment wear. It is difficult to maintain effective sealing to prevent escape of gas from the contacting zone and equipment maintenance costs are high. These problems are magnified when solids or gas or both are at elevated temperatures. Pressure lock systems having on-off valves through which the solids pass have been the preferred method of feeding solids into a pressurized zone, but the valves are a high-maintenance item. A system using valves is briefly described below.
U.S. Pat. No. 2,851,401, cited above, discusses the problems involved in solids transfer and teaches transfer of solids without using mechanical equipment subject to wear or which degrades solids. However, this patent does not deal with the various aspects of gas flow, such as mentioned above. Also, it is often desirable to maintain a continuous as flow, even when the solids flow is batchwise. Use of continuous gas flow permits better control of contact time period and usually promotes the chemical or physical process taking place by constantly presenting fresh gas to the solids. In some cases, it is highly important to immediately contact incoming solids with fresh gas, that is, gas which has not yet had significant contact with solids.
The present invention is useful in the practice of a variety of processes and, in particular, in hydrocarbon conversion processes, such as catalytic reforming, which is the subject of the detailed example presented below. Another process in which the invention may be utilized is the conversion of C.sub.3 /C.sub.4 LPG to light olefins. This catalytic dehydrogenation process will convert, for example, propane to propylene. In another catalytic hydrocarbon conversion process, LPG is processed to yield aromatics and hydrogen. The present invention is useful in regenerating the catalyst used in these processes. An example of a process other than hydrocarbon conversion in which the present invention may be applied is the treatment of a gas stream to remove a component by means of contact with particulate solids, such as removal of sulfur dioxide from a flue gas stream upon passing the flue gas through a bed comprising a sulfur oxide acceptor such as copper-bearing alumina spheres. However, the preferred use of the invention is in hydrocarbon conversion processes and specifically in moving bed catalytic reforming.